Question: $ D = \left[\begin{array}{rrr}2 & 2 & 0 \\ 2 & 0 & 0 \\ 0 & 0 & 1\end{array}\right]$ What is $ D^{-1}$ ?
$ = \left[\begin{array}{rrr}0 & \frac{1}{2} & 0 \\ \frac{1}{2} & -\frac{1}{2} & 0 \\ 0 & 0 & 1\end{array}\right]$